Method of annealing and cleaning ferrous articles



Patented Feb. s, 1935 METHOD OF ANNEAIJNG AND CLEANING FERROUS ARTICLESGustav A. Reinhardt, Youngstown, Ohio, assignorto The Youngstown Sheetand Tube Company, Youngstown, Ohio, a corporation of Ohio No Drawing.

3 Claims.

My invention relates to the treatment of ferrous articles in the courseof their manufacture and more particularly to an improved method forsimultaneously cleaning and annealing ferrous 5 sheets and plates priorto their being coated with zinc or tin although my invention may alsoadvanta'geously be employed for substantially similar purposes in themanufacture of other ferrous articles or shapes and at other stages inthe course of manufacture.

In the preparation of iron and steel sheets, plates and other ferrousarticles for galvanizing or tinning, ithas heretofore been customary tofirst anneal the articles in a furnace by either of the followingwell-known processes, box annealing, open annealing or normalizing, andthereafter pickle them by introducing them into an acid solution, theannealing being efiected for the purpose of obtaining the desiredductility and grain structure and the pickling to remove scale and otherforeign matter from the surface of the metal so as to enable asatisfactory coating to be subsequently applied thereto. My invention isdirected to a method by which these two operations may be combined andthe annealing and cleaning accomplished by a single step or operationand as it is particularly useful when employed for treating iron andsteel plates and sheets just prior to galvanizing or tinning, it willherein be more particularly described with reference thereto.

When in the ordinary course of ferrous sheet manufacture the sheet hasbeen rolled to the desired thickness, its grain structure is such that arearrangement thereof is usually desired and is ordinarily effected bybox annealing or other equivalent process preparatory to pickling byimmersion in an acid bath. At this stage in accordance with my improvedmethod and instead of annealing them in a furnace, I introduce thesheets into a basic bath comprising molten sodium carbonate maintainedat a temperature above its melting point, 852 C. After the sheets havebeen immersed in this bath a short time, for example, with ordinarysheets such as are used for tin plate, about fifteen seconds, they areremoved from the bath, cooled, then washed in or sprayed with water andallowed to dry. After treatment in the manner described the sheets arefound to be rearranged and refined as to grain structure similarly tothe rearrangement eifected by furnace annealing and are, additionally,free of scale and other surface impurities, which, if present, wouldinterfere with subsequent tinning or galvanizing.

Application May 11, 1932, Serial No. 610,708

- In accordance with my method when a lower temperature bath is desireda small quantity of sodium hydroxide may be added to the sodiumcarbonate, thus somewhat reducing the melting point of the latter andpermitting it to be maintained in a fluid state at a relatively lowtemperature.

The sheets upon their removal from the molten bath, when treated inaccordance with the method just described, carry a thin coating ofsodium ferrate, resulting from the reaction of sodium carbonate with theiron oxide scale usually present before the treatment together'with anouter film of the sodium carbonate, which prevents oxidation of thesheets during the subsequent cooling. Both of these compounds, beingsoluble in water, are readily removed from the sheets by washing so thatthe sheets are thereafter perfectly clean, well descaled, brighter thanan ordinary pickled sheet and more rust-resistant. Moreover, whereasacid pickling usually results in absorption of hydrogen by the sheetswith resultant defects in the subsequent coating when the sheets aregalvanized or tinned, sheets treated by my method are substantially freeof hydrogen and therefore-may be coated with zinc or tin without theblistering frequently encountered in the case of pickled sheets andwhich is thought to be due to expansion of this occluded hydrogen underthe influence of the heat of the galvanizing or tinning operation.

It will, of course, be appreciated that since molten sodium carbonatecauses rapid corrosion of certain metals, it is necessary to use forholding the bath a pot made of some material which is not readilyattacked thereby and I have found nickel and certain alloys thereofparticularly useful for this purpose, since they are generallysubstantially inert to the action of the compounds I employ, even whenin a molten state.

For the most successful and satisfactory practice of the invention, thearticles being treated should be heated slightly above the thermalcritical point or temperature of the metal of about 900 C.; in the caseof low carbon steel, such as is commonly used for the manufacture ofsheets and plates, this result is readily accomplished by the use of abath of soda ash or sodium carbonate as above described since themelting point thereof, namely 852 C. is very slightly below the thermalcritical point or temperature of such steel. Moreover, an importantadvantage of the invention resides in the fact that while the article isin the bath it is subjected to a uniform temperature at all points sothat the granular strucwill subsequently have substantially uniformcharacteristics as to grain refinement, while as less metal is removedfrom the surface than by the usual pickling operation, a considerablesaving of metal in the aggregate is accomplished.

while I am aware it has heretofore been suggested that in the treatmentof sheets prior to the galvanizing or tinning it might be desirable tosubject them to the action of a bath consisting of a molten mixture ofsodium chloride, sodium carbonate and sodium cyanide, this practicewould be highly dangerous and thus impractical for use on a commercialscale because highly poisonous fumes are given off by such a bath due tothe presence of the cyanide, which result in serious or even fatalpoisoning of operatives in the vicinity of the bath, whereas when thebath of my invention is employed, no poisonous fumes or gases of anycharacter are given off and the safety of the operatives is thusunaffected.

By my invention not only is the annealing accomplished substantiallyinstantaneously, as distinguished from the several hours heretoforerequired for box annealing, but, additionally, the necessity forproducing the relatively expensive pickling machines heretofore used isavoided and the costs involved in acid pickling of the sheetssubstantially eliminated. It is thus evident that by the use of mymethod very substantial savings in ultimate manufacturing costs may beeffected, the operations corresponding to annealing and pickling in themethods heretofore used being accomplished relatively rapidly with theaid of apparatus of very substantially less cost both initially and formaintenance than that theretofore used to effect corresponding results.Furthermore, the output of a plant may be appreciably increased by theelimination of pickling losses, it being well known in the industry thatthe iron dissolved from sheets of the character under consideration bythe ordinary acid pickling ture throughout the entire extent of thearticle bath constitutes a relatively large percentage of their totalweight, whereas when my invention is employed, substantially no loss offerrous metal whatever is occasioned.

While I have herein described my invention with some particularity as itmay be utilized in the manufacture of sheets preparatory to coating themwith protective materials, it will be understood that it may be equallywell employed for the treatment of other kinds of sheets and plates aswell as other ferrous shapes and manufactured articles generally andthat changes and modifications in the details thereof will readily occurto those skilled in the art and may be made if desired without departingfrom the spirit and scope of the invention as defined in the appendedclaims.

Having thus described my invention, I claim and desire to protect byLetters Patent of the United States;

1. A method of annealing ferrous articles which comprises the steps ofimmersing the articles in molten sodium carbonate, maintaining thearticles submerged in the bath for a predetermined time less than oneminute, and thereafter removing the articles and allowing them to cool.

2. A method of annealing a ferrous sheet prior to coating it with metalwhich comprises the steps of dipping the sheet in a molten bath ofsodium carbonate of slightly higher temperature than the criticaltemperature of the sheet metal, maintaining the sheet in the bath forapproximately fifteen seconds, thereafter removing and cooling thesheet, and finally washing it to thereby remove a surface coatingresulting from the bath and adhering to the sheet.

3. In a method of simultaneously cleaning and annealing a ferrous sheet,the step of immersing the sheet in a bath of molten sodium carbonatemixed with molten sodium hydroxide for a period of time not exceedingone minute.

GUSTAV A. REINHARDT.

